This article is the most I've seen on Joss' involvement on Toy Story. See section II.

How much time altogether did you end up investing in the project?
Joss: More than six months. It was not a polish; it was a rewrite, and with animation you’re writing with every visual. Every shot is up on a board somewhere, so you’re writing in great detail.

and

I went up to Pixar [the Northern California-based animation studio which produced “Toy Story”], and stayed there for weeks and wrote for, I think, four months before it got greenlit, and completely overhauled the script. There was some very basic things in there that stayed in there. The characters were pretty much in place except for the dinosaur, which was mine.